BISC 421 1st Edition Lecture 20 Outline of Current LectureI.Visual Cortces Current LectureVisual CorticesThe Projecton of the Visual Field on the RetnaDescartes, Le Dioptrique1637, after KeplerThe lenses of the eye (like typical lenses) focus light on the retna in such a way that the projecton of the visual scence is upside and backwards.•Concerned that the optics of the eye were distorting his measurements-‐ wanted to solve the optics of the eye •The world is projected on the retina upside down and backwards.Monocular and Binocular Visual Fields1. The visual fields projected on the retna before the lens flips the worldupside down and backwards.2. Most of the visual field is seen by both eyes. This is labeled, binocular visual field.3. The visual fields projected on the retna after inversion by the lenses.•Monocular-‐ one eye binocular-‐ two eyes•Four quadrants of the visual field•See most of the world with both eyes•Would lose some of the periphery if you cover one eye •Images are upside down and backwards when projected on the retna.The Contralateral Side of the Brain Sees the Ipsilateral Visual Field to the left thalamus (LGN)The partal decussaton of the optc nerve ensures that each half of the brain receives input from one full hemifield -- the right hemifield is “seen” by the left side of the brain and the left hemifield by the right side.•Sliced at the azimuth•Image falls on both retnas at the fovea•We want to match all of the maps of the body together•Axons from the temporal retna stay on the contralateral side whereas the nasal retna axons cross •This is called partal decussation.Ganglion cells project to the Thalamus: Relay cells project to Primary Visual Cortex1. Retnal axons travel from the eye to the thalamus. Relay cells in thalamus project to the cortex.2. The myelinated axons of relay cells form a thick stripe in the primary visual cortex, akastriate cortex, occipital cortex, V1 or area 17.•Schematc image of the human visual system•These structures project to the LGN•The striate cortex are the axons of the fibers coming from the thalamus•This is only in the primary visual cortex •This is a horizontal secton of the brain.Retnotopic Organizaton of Primary Visual CortexThe foveal region is caudalmost, the superior visual field is ventral and the inferior visual field is dorsal.•Sagital secton of the brain•Same order in the retna is preserved in the thalamus and the visual cortex •Don't need to remember all of these structuresRetnotopy and the Cortcal Magnificaton FactorVisual Stmulus •Could see the map of the visual stmulus in the brain itself•Can see the cortcal magnificaton from this mappingGross and Microscopic Anatomy of the CortexCortex has 6 layers. 1 is on the top (near the pia) and 6 is on the botom (near the white mater). Cortex is built of narrow cylindrical arrangements of connected cells that cross each of the 6 layers. Axons from the thalamus enter via layer 4.•These are the layers of the cortex-‐ can see the relatve density of the layers•Columns of cells in layer 6•Cortex is made of columns of cells•Aferents from LGN come into layer 4Orientaton Selectvity•The animal is looking at light projected on a screen•Recording from a neuron that is just sensitve to one region of spaceSpike count•Swept bars across a screen in various directions•Count the number of acton potentals that a neuron fired when it looked at thesevarious stimuli•This cell in partcular really likes vertcal bars-‐ preferred orientaton•Basically countng the number of spikes•This is called a tuning curve for a partcular cellConcept of Functonal ArchitectureOrientaton Column, vertcally aligned cells across the cortcal depth (the different layers) share the same preference for stmulus orientation.Hypercolumn, region that represents one point in visual space (receptive fields are overlapping) and in which a given stmulus atribute e.g. Orientation, directon of moton etc. is analyzed for all permutations.•Electrode put down through the layers in a supposed mini column•Find that the receptve fields overlap and find that the tuning curves overlap-‐ have the same preferred orientation•Put an electrode through the top layers 2 and 3 and found that there is a progressionof thereceptve filed through visual space-‐ have a mapping of all orientatons•This is not perfect but it is good•The cells have diferent preferred orientations•Receptve fields that are well overlapped in visual space may encode very diferent directions- ‐idea of the hypercolumn-‐ region that represents one region in visual space in which there can be multple orientatons mapped.Animaton of hypercolumn, illustratng orientaton columns as an example1o of visual space Hypercolumn (~1 mm2 of cortex)representng 1o of visual space. The colorof the bars indicate the orientaton column thatis actvated by the corresponding bar at left.Orientaton is but one of many parameters that are mapped in the hypercolumn•This is a hypercolumn that represents one region in visual space•Each cell responds for whichever orientaton column it codes for•One chunk of cortex sees one region of visual space but it is made up of a lot ofminicolumns that together form full percepton.Radioactve tracer was injected in one eye and transported all the way to the visual cortex. The dark bands are those that were labeled by the tracer.Ocular Dominance Columnstangental view coronal view•The ocular dominance column: dark bands get input from one eye and light sectons from both eye•Not untl cortex that you get a fusion of binocular inputs-‐ in LGN its monocularArrangement of Ocular Dominance Columns within the HypercolumnOcular dominance is important for our ability to judge depth.•Some cells like input from one eye or the other or they like input from bothThe Ice Cube model of the Functonal Architecture of VIsome features contained within a single HYPERCOLUMNorientatondirectonretnal disparity (for stereopsis)spatal frequency (for acuity)•There are a lot of features that are mapped in a hypercolumn•Hypercolumns analyze all parameters of each of these features•Look at animaton on black boardTwo Different Visual Streams Begin in Retna•P cells are small-‐ inputs from cones•M cells-‐ large receptve field, fast, detecton of movement, form visionThe Parvo & Magno Streams are Segregated in the Thalamus and